Photoplethysmography (PPG) is the optical acquisition of a plethysmogram, which is the measurement of a volumetric variation of tissue. PPG often relates to variations of blood volume so that PPG may sometimes be interpreted as PPG of blood volume.
A conventional reflective PPG sensor monitors perfusion of blood to the dermis and subcutaneous tissue of skin through absorption measurement at a specific wavelength. Besides light originating from blood there is a far greater portion detected originating from tissue and or blood slushing.
FIG. 1 shows two typical conventional PPG implementation forms. The left portion of FIG. 1 shows an example of a reflective implementation with an optical transmitter 10 (e.g. light emitting diode (LED), laser diode etc.) and an optical receiver 20 (e.g. photo detector, photo diode, photo transistor etc.). Light emitted by the optical transmitter 10 is reflected at a skin portion of a finger tip 100 of a patient or user and then received as a PPG signal at the optical receiver 20. The right portion of FIG. 1 shows an example of a transitive implementation where light emitted by the optical transmitter 10 is transmitted through a tissue portion of an ear 110 of a patient, and then received as a PPG signal at the photo detector 20 as optical receiver.
PPG signals contain a very small alternating current (AC) signal component (the actual plethysmogram) on top of a very large unwanted offset, usually and incorrectly called the DC (direct current) signal. This DC offset usually comprises a low frequency (LF) component consisting of a large portion of back-scattered light not originating from blood of the patient or user, ambient light (if not filtered), and variations of both caused by e.g. motion. The frequency components of the LF component can include the same frequency as the AC signal to be measured, thereby excluding any frequency domain filtering. A conventional PPG sensor therefore typically measures both signals together and uses signal processing algorithms to separate the different components.
The above components of the PPG signal lead to the problem of a reduced usable dynamic range, since major part of the signal processing resolution (e.g. analog-to-digital converting (ADC) etc.) is wasted on sampling the unwanted DC and LF components.
Moreover, motion introduces large variations in the unwanted DC and LF components, which causes major problems in PPG sensors due to motion artefacts. Therefore, additional motion sensors (e.g. three dimensional (3D) accelerometers, or additional optical sensors) are used for artefact suppression in conventional PPG sensors, which leads to increased complexity of PPG sensor devices.
Conventional proposals to overcome the above problems are mostly based on measuring actual movements and compensate the measured PPG signal (which hopefully includes the same motion artefacts) based on some pre-determined correlation algorithm. Mostly, 3D accelerometers have been suggested as suitable motion sensors, but the use of additional pseudo-PPG sensors has also been proposed. These pseudo-PPG sensors use e.g. an infrared (IR) wavelength (where blood is known to have (almost) no absorption), but skin tissue has. These pseudo-PPG sensors thus measure a kind of motion artefact which is used to compensate the unwanted motion artefacts in the actual PPG sensor. The problem with this kind of compensation is that the more the secondary wavelength of the pseudo-PPG signal differs from the primary wavelength of the actual PPG signal, the larger the differences are in the optical parameters of the tissue through which the light propagates. For example, IR light will propagate much deeper into the tissue than the shorter primary wavelength. Therefore, the ‘motion’ sensed by the secondary wavelength of the pseudo-PPG signal is based on a considerably different volume than that of the primary wavelength of the actual PPG signal. This requires additional compensation, and thus increases complexity.
Furthermore, compensation using 3D sensors has the disadvantage that they are expensive, relatively large and use additional supply current. Again, complex compensation algorithms have to be implemented, which requiring more process power.
Thus, in conventional PPG sensors the unwanted DC and LF components are removed by suitable signal processing after A/D conversion. A conventional PPG sensor therefore typically measures both signals together and uses signal processing algorithms to separate the different components.
As another example of conventional PPG sensors, US2003/036685 discloses a physiological signal monitoring system where two PPG sensors with optical radiation sources are used for providing two different functions. First, they are used as SpO2 sensor, where two separated wavelengths (red and infrared) are used to estimate Hb oxygen saturation (i.e., SpO2). One wavelength (880 nm) were the absorption coefficients are nearly equal, and one (658 nm) where the absorption coefficient differ greatly. Second, they are used as pulse wave velocity sensors based on cross correlation between the two output signals.